The present invention relates generally to the art of compressing a gas in an oil-injected rotary screw compressor. More specifically, the present invention relates to the prevention of the high speed reverse rotation of screw rotors due to the backflow of gas from the high pressure portion of a screw compressor refrigeration circuit, through the compressor, to the low pressure portion of the circuit upon compressor shutdown.
Compressors are employed in refrigeration systems to raise the pressure of a refrigerant gas from a suction pressure to a higher discharge pressure which permits the ultimate use of the refrigerant to accomplish the cooling of a desired medium. Many types of compressors, including reciprocating, scroll and screw compressors, are used in refrigeration applications. Screw compressors employ complementary male and female screw rotors disposed within the working chamber of a rotor housing to compress gas. The working chamber can be characterized as a volume generally shaped as a pair of parallel intersecting cylindrical bores closely toleranced to the outside length and diameter dimensions of the screw rotors disposed therein. The screw rotor housing has low and high pressure ends which include suction and discharge ports respectively. Both the suction and discharge ports are in flow communication with the working chamber of the rotor housing.
Refrigerant gas at suction pressure enters the compressor working chamber via the suction port at the low pressure end of the rotor housing and is there enveloped in a pocket formed between the rotating complementary screw rotors. The volume of this chevron-shaped pocket decreases and the pocket is displaced toward the high pressure end of the compressor as the rotors rotate and mesh within the working chamber. The gas within such a pocket is compressed by virtue of the decreasing volume in which it is contained, until the pocket opens to the discharge port at the high pressure end of the compressor. As the pocket opens to the discharge port, the volume of the pocket continues to decrease and the compressed gas is forced through and out of the discharge port of the rotor housing.
Due to the extremely close tolerances between the rotors of the screw rotor set and the elements in the compressor which cooperate to define the working chamber in which the rotors are disposed, the bearing arrangement by which the rotor set is mounted within the working chamber is critical to compressor operation and life. The bearings in a screw compressor are subject to high axial and radial loads which can vary greatly from the low pressure end of the compressor to the high pressure end. Protection and lubrication of the rotor bearings is therefore of paramount concern in the design of rotary screw compressors. Since the suction and discharge ports of screw compressors are valveless and are essentially unobstructed openings into and out of the working chamber of the compressor, the rotor set within the working chamber is exposed, in operation, to the high pressure gas downstream of the compressor discharge port. Additionally, the gas undergoing compression in a pocket between the rotors bears against the high pressure end wall of the working chamber to create additional thrust on the rotors in a direction toward the low pressure end of the compressor. Therefore, a large axial thrust is developed, in operation, on the rotor set of a screw compressor in a direction from the high pressure end of the compressor to the low pressure end. This axial force must be compensated for by the compressor's bearing arrangement.
At compressor shutdown, the backflow of high pressure gas from the high pressure side of a refrigeration system through the open discharge port of the compressor toward the low pressure side of the system, if allowed to occur, would cause the high speed reverse rotation of the no longer driven screw rotors within the working chamber. Such freewheeling of the rotors could occur at speeds greater than the maximum design RPM of the rotor set and rotor bearings. Additionally, the resulting rush of downstream high pressure gas back through the compressor to the low pressure side of the system could result in a surge of pressure into the low pressure side of the system such that a higher pressure might momentarily develop at the suction end of the compressor than exists at the discharge end of the compressor. This situation would result in a re-surge of pressure and gas from what is normally the low pressure side of the system to what is normally the high pressure side of the system in attempt to equalize system pressures and would further result in inordinate and uncommonly large axial forces acting on the screw rotor set and rotor bearings in a direction opposite that normally expected and compensated for in operation. That is, axial force will be brought to bear on the screw rotor set and the bearings in which the rotors are mounted in a direction toward the high pressure end of the working chamber of the compressor. Several untoward results can occur if such high-speed reverse direction rotor rotation and the resulting pressure transients are allowed to occur. Among these results are the aforementioned development of axial thrust on the rotor set in a direction which is not compensated for to the degree normal axial thrust is compensated for within the compressor. Further, possible mechanical failure due to the achievement of rotor speeds exceeding design RPM might occur. Additionally, most, if not all, screw compressor bearing lubrication schemes are predicated on the development of pressure downstream of the compressor to drive lubricating oil to the rotor bearings. The high speed reverse rotation of the rotor set and momentary development of high pressures upstream of the working chamber, if allowed to occur, could theoretically cause oil to be sucked from the bearings or, in any event, not to be delivered to the bearings with possibly catastrophic results.
The number and complexity of patented screw compressor bearing and/or bearing lubrication schemes illustrates the ongoing need for an uncomplicated, inexpensive device by which bearing and/or bearing lubrication arrangements in screw compressors can be protected and simplified.